A high power, solid state amplifier, when driving uncertain load impedances having a high voltage standing wave ratio (VSWR), may suffer from serious degradation in performance due to mismatched loads on the amplifier. For example, a radio frequency (RF) amplifier driving a transmission system with 50 ohms nominal impedance may be required to provide full forward power output into a load with a 3:1 VSWR. A class B amplifier operating under these conditions suffers from thermal and electrical stress, such as increased junction temperatures in its transistors, gain compression, and input mismatch conditions to the driver stage. Elevated junction temperatures in the solid state devices caused by high power dissipation in excess of that under normal operating conditions seriously reduces the device's reliability, with the resulting stress permanently damaging the critical solid state devices.
One technique for mitigating the adverse effects of abnormal load impedances from a high VSWR load is isolating the load. An example of this technique involves placing a magnetic isolator at the output of each solid state device. Under high VSWR conditions, power reflected by the load is absorbed in a dump port termination. Other techniques use a VSWR detection circuit to monitor the load status and provide a control signal to initiate the reduction of the power output of the amplifier to a safe operating value.
In high power systems requiring constant forward output power, these approaches are not feasible because a constant forward output power from the amplifier is not maintained. An approach that protects the solid state components from high power dissipation but that also maintains full power output substantially independent of the load VSWR is most desirable.